Structures and production methods of retroreflective articles such as retroreflective sheets comprising a plurality of reflecting beads which are fixed in the support layer having a raised surface are disclosed in several publications. JP-A-55-65524 and JP-A-57-193352 disclose methods including the steps of applying a foaming component to a flat substrate surface in a spot form, and coating a resin over the substrate surface to make a support layer. On the support layer, transparent glass beads are scattered, and then the paint for the support layer is dried and cured to fix the glass beads to the support layer. Finally, the substrate is heated to blow the foaming component to produce projections.
Publications JP-A-53-46363 and JP-A-5346371 disclose methods including embossing a substrate made of thermoplastic resins to make a raised surface, and coating a paint containing resins in a thickness sufficient for filling indented portions to obtain a support layer having a flat surface. Transparent glass beads are embedded in the surface of the support layer, and then the resin in the support layer is cured to fix the glass beads in the layer. Finally, the whole sheet is heated so that the surface of substrate is recovered from the flat state to form raised surface having varying heights corresponding to the difference of coated thicknesses of the substrate resins.
Publication JP-A-57-10102 discloses a method including coating a paint which shrinks and forms wrinkles upon drying on a substrate, scattering glass beads over the wet paint, and then heating and drying the paint to form projections to which the glass beads are fixed.
Publications U.S. Pat. No. 4,069,281 and JP-A-58-237243 disclose retroreflective sheets that are produced by making projections of a paint containing resins on a substrate, embedding glass beads in the projections before drying and curing the projections, and then curing the projections to fix the glass beads.
U.K. Patent 2,251,091 discloses a retroreflective sheet produced by adhering transparent glass beads to an aluminum layer which has a plurality of indentations.
Publications JP-A-58-208041 and JP-B-7-84726 describe methods for producing the retroreflective sheets including the steps of forming a support layer from a thermoplastic polymer, and embedding parts of transparent glass beads in the surface of the support layer. Then, the support layer is embossed from the side on which the beads have been embedded, and the projections in which the parts of beads are embedded, and also the depressions in which the beads are entirely embedded.
The methods and structures described in the foregoing references do not use any light-transmitting covering layer which covers the surfaces of the beads.
Publication JP-Y-62-41804 discloses a reflective sheet having projections which are intended to prevent sticking bills thereto. This reflective sheet is produced, for example, by adhering a retroreflective sheet having a light-transmitting covering layer which covers the surfaces of beads to the surface of a substrate and embossing the substrate from its back face to form projections on the surface of the substrate. However, in general, the size of the projections which are formed to prevent sticking bills is relatively small, for example, a width of 2 mm and a height of 1 mm. Furthermore, this utility model publication does not teach any favorable reflective characteristics for incident light in a wide incident angle from a low incident angle (a direction close to the normal line to the reflective plate) to a high incident angle (a direction close to the reflective plate), that is, sizes and arrangement of the projections for a reflective sheet having good wide incident angle reflective characteristics.
Publications WO97/01677 and WO97/01678 disclose retroreflective materials comprising reflective wall-form raised portions and flat areas which are covered with an enclosed lens type retroreflective sheet, although they are not produced by embossing. However, these publications do not suggest the improvement for easily removing water droplets and solid foreign substances adhered to the flat areas.
In view of the relative disadvantages of the methods and structures described above, it is important for reflective projections to have predetermined sizes and patterns for the formation of a plate-type reflective material with excellent wide incident angle reflective characteristics. This is particularly important for the improvement of reflective characteristics against light having a relatively high incident angle, for example, 70 degrees or higher from the normal line to the reflective surface.
The reflective plates should have properties suitable for outdoor use, when they are used as construction parts of signs used outdoors. That is, they are required to have properties that their reflective luminance hardly decreases when water droplets adhere to the reflective plates, that the foreign substances such as dust are easily removed if they adhere to the reflective plates, and that they maintain sufficient reflective luminance during use.
An object of the present invention is to provide a wide incident angle reflective plate which has both good wide incident angle reflective characteristics and outdoor use characteristics.